Fe3O4/SiO2 decorated trimesic acid-melamine nanocomposite: a reusable supramolecular organocatalyst for efficient multicomponent synthesis of imidazole derivatives

This article describes supramolecular Fe3O4/SiO2 decorated trimesic acid-melamine (Fe3O4/SiO2-TMA-Me) nanocomposite that can be prepared with features that combine properties of different materials to fabricate a structurally unique hybrid material. In particular, we have focused on design, synthesis and evaluation a heterogeneous magnetic organocatalyst containing acidic functional-groups for the synthesis of biologically important imidazole derivatives in good to excellent yields. The introduced Fe3O4/SiO2-TMA-Me nanomaterial was characterized by different techniques such as FTIR, XRD, EDX, FESEM, TEM, TGA and DTA. As a noteworthy point, the magnetic catalytic system can be recycled and reused for more than seven consecutive runs while its high catalytic activity remains under the optimized conditions.

www.nature.com/scientificreports/ in the three-component synthesis of imidazole derivatives from benzil (2) or benzoin (3), aldehydes (4), and ammonium acetate (5, Fig. 2). To the best of our knowledge, there is not any report for the use of pseudosupramolecular heterogeneous magnetic organocatalyst having acidic functional groups for the synthesis of imidazole derivatives.  (1) presented a very strong and broad band, covering a wide range between 2800 and 3600 cm -1 , for the O-H stretching vibrations of the carboxylic acid functional groups as well as Fe 3 O 4 /SiO 2 . Furthermore, the signals at 1730, 1710 and 1683 cm −1 are assigned to the carbonyl groups of ester, acid and amide, respectively. It should be noted that the presence of carbonyl group of ester indicates the formation of a covalent bond between the acid groups of trimesic acid and the magnetic core/shell. Furthermore, the asymmetric vibration signals of Si-O-Si and Si-OH as well as the symmetric vibration signal of Si-O-Si could be seen at 1090, 930 and 790 cm -1 . In addition, the characteristic band for Fe-O stretching vibrations was observed at 560 cm −1 .

Results and discussion
The morphological features and particles size of the new magnetic nanocomposite Fe 3 O 4 /SiO 2 -TMA-Me nanocomposite (1) were examined by FESEM and TEM experiments (Figs. 4 and 5). The catalyst nanoparticles are approximately spherical and have been distributed with an average diameter of about 75 nm. On the other hand, TEM images ( Fig. 5) obviously demonstrate decoration of core/shell magnetic nanoparticles on the trimesic acid/melamine rod-shaped structure. Also, the TEM images can be considered as a confirmation of the pseudo-supramolecular structure. Indeed, by considering this point that the Fe 3 O 4 /SiO 2 is a core/shell structure, according to its preparation method, the rod-shaped particles shown in FESEM images may be attributed to the polymerized structure of melamine and trimesic acid.
The Energy dispersive spectroscopy (EDX) of the Fe 3 O 4 /SiO 2 -TMA-Me (1) is shown in Fig. 6. The EDX spectrum indicates that the introduced nanocatalyst 1 is composed of Fe, O, N and C elements.
Also, Fig. 7 Fig. 8 show that the slight weight loss between 35-150 °C can be assigned to the elimination of adsorbed solvent or water molecules on its surface or trapped inside of the sample. Also, the weight losses between 150-270 °C and   Table 1.
The results of using different conditions in model reaction have been presented in Table 1. It is noteworthy that a very low yield of the desired product 6a was obtained in the absence of the Fe 3 O 4 /SiO 2 -TMA-Me (1) ( Table 1, Entries 1-4). By using different solvents, the best result was obtained with ethanol at room temperature. In the next step, the amount of catalyst loading was optimized (Table 1, Entries 9-13). Although the reaction time using 15 or 20 mg of the catalyst loadings is slightly less than compared to 10 mg loading in EtOH under reflux conditions, no noticeable change in efficiency was seen. For this reason, the optimal amount of catalyst was   Table 2. Noticeably, the desired products 6a-m were obtained in high to excellent yields. The obtained results obviously confirm the applicable catalytic activity of the Fe 3 O 4 /SiO 2 -TMA-Mel nanomaterial (1) to promote the three-component condensation of a wide range of aldehydes with benzil or benzoin and ammonium acetate (Supplementary Figure S1).
According to above results presented in Table 2, the following mechanism can be proposed for the synthesis of imidazole derivatives 6 by starting from benzil (2) or benzoin (3) catalyzed by nanocatalyst 1 (Fig. 9). First,   (1) is that it can be magnetically separated from the reaction mixture after each run, collected, washed using acetone and n-hexane, respectively, and then reused in the subsequent model reactions. The model reaction was performed using the recycled catalyst for several times. As a result, a slight decrease in the catalytic efficiency was observed after the seventh run (Fig. 10). TEM and FESEM images as well as XRD pattern of the reused heterogeneous catalyst Fe 3 O 4 /SiO 2 -TMA-Me (1) have been presented in Fig. 11, which show excellent stability of the catalyst 1 under optimized reaction conditions.
To evaluate the efficiency of Fe 3 O 4 /SiO 2 -TMA-Me acidic catalyst (1), a comparison has been made with the previously reported methods for the synthesis of imidazole derivative 6a. As shown in Table 3, the prepared nanocatalyst can compete with the similar systems in terms of catalyst loading, reaction conditions, and catalyst reusability times.    The mixture of trimesic acid (TMA, 3 mmol), 1-hydroxybenzotriazole (HOBT, 3 mmol) and 1-ethyl-3-(3dimethylaminopropyl)carbodiimide (EDCI, 3 mmol) was stirred in deionized water/acetonitrile (1:1, 50 mL) for 30 min, then 1 mmol of melamine was added and the obtained mixture was stirred for 24 h at room temperature. After this time, 0.3 g prepared Fe 3 O 4 /SiO 2 was mildly added and stirred for 24 h to afford the final precipitate. Afterward, the obtained solid was collected with an external magnet, washed several times using distilled water and EtOH (96%) and then dried at 45 °C for 3 h.  Table 2. After completion of the reaction, the catalyst 1 was separated by an external magnet. Afterwards, H 2 O was added drop wise into the solution until imidazole derivatives 6 were completely precipitated. The obtained mixture was filtered off and the precipitate were washed and then dried in an oven at 70 °C for 1 h. The recycled catalyst 1 was washed with acetone and n-hexane (1 mL), respectively and then dried at 50 °C for 2 h and stored for another run.   www.nature.com/scientificreports/

Conclusion
The magnetic Fe 3 O 4 /SiO 2 decorated trimesic acid-melamine pseudo-supramolecular (Fe 3 O 4 /SiO 2 -TMA-Me) nanocomaterial was prepared and properly characterized for the first time. The Fe 3 O 4 /SiO 2 -TMA-Me nanocomposite was used for the three-component condensation of benzil or benzoin, aldehydes, and ammonium acetate to afford the corresponding imidazole derivatives. Low catalyst loading, high to excellent yields of the desired products, easy and quick isolation of the products from the reaction mixture as well as reusability of the solid acidic pseudo-supramolecular nanocomposite with negligible loss of its activity are the main advantages of this method. In addition to the catalytic applications, other applications of this nanomaterial, as a pseudosupramolecular structure, are ongoing in our laboratory and would be presented in due course.

Data availability
The datasets generated and/or analyzed during the current study would be available in the Science Data Bank repository after acceptance of the manuscript.